The invention relates to a digital low-pass filter comprising m series-arranged registers, a digital input signal, which is present as a sequence of amplitude-discrete sampling values having a sampling frequency determined by a clock signal, being applied to the first of these registers, multipliers multiplying by switchable coefficients subsequences of (m+1) sampling values at the input of the first register, and the outputs of the registers and a first adder to which the output signals of the multipliers are applied.
Such a low-pass filter is disclosed in the periodical "Elektronik", 14./15.07.1983, pages 61 to 64 and is used in the digital-to-analog conversion of audio signals. The signals are entered into the low-pass filter at a frequency of 44.1 kHz and read at a frequency of 176.4 kHz. The word-length of the 16-bit input signal is increased to 28 bits using multiplication by 12-bit coefficients in the low-pass filter. In a subsequent noise shaper the signal is rounded to 14 bits, the round-off error is added to the input signal of the noise shaper via a register and the rounded-off output signal is applied to a digital-to-analog converter. The output signal of the circuit arrangement has a greater signal-to-noise ratio than an output signal from a 14-bit digital-to-analog converter without low-pass filter and without noise shaper.
Digital video signal processing requires a sampling frequency which does just satisfy the sampling theorem, in order not to increase, for example, the cost of delay elements unnecessarily. The current analog-to-digital converters appropriate to this object which operates at, for example, a chrominance carrier frequency of 4.43 MHz with a sampling frequency of 13.5 MHz produce odd harmonics of the chrominance sub-carrier which negatively influence the luminance signal. More specifically the third harmonic of the chrominance sub-carrier, located at 30.3 MHz, is annoying, as it causes interference in the luminance signal also at 0.2 MHz because of the periodicity of the spectrum. The interference in the third harmonic of the chrominance sub-carrier can be avoided by increasing the sampling frequency of the analog-to-digital converter. So as not to increase the cost and design effort of the subsequent digital circuits, the prior art digital low-pass filter can be added with the object of reducing the sampling frequency. An advantageous sampling frequency for the analog-to-digital converter is obtained at a frequency of 20.25 MHz. In a further signal processing operation using the 13.5 MHz sampling frequency the digital low-pass filter must provide a reduction by a factor of 1.5.